Heavy vehicles are commonly supplied with disc brakes that are liquid cooled; and in the case of large earth moving vehicles, such as tractor scrapers, the rear and front brakes may have individual cooling systems due to the distance between them. In addition, such vehicles commonly are arranged so that their rear brakes may operate independently of the front brakes when they are being used only to retard vehicle speed, as on a long hill. With independent cooling systems, each must have the capability of dissipating the heat generated by the respective brakes during operation; and this has commonly required that the cooling system for the rear brakes have a much higher capacity than that for the front brakes. This causes the cooling systems for the brakes of such vehicles to be quite expensive and complex.